Staphylococcus aureus, a gram-positive, spherical bacterium, is a common colonizer of the human skin and mucosa. It causes skin and wound infections, urinary tract infections, pneumonia and bacteremia. It is also commonly implicated in food poisoning. It is also found as a common contaminant in pharmaceutical and cosmetics products (4).
Vogel-Johnson Agar is prepared according to the formula devised by Vogel and Johnson (1) and is recommended for the microbial limit test in USP (2). Originally it was developed by Zebovitz (3), as a Tellurite Glycine Agar, a selective medium for the detection of coagulase-positive staphylococci. Vogel-Johnson modified the medium in 1960 by the addition of phenol red as a pH indicator and by increasing the quantity of mannitol (1). Selection and differentiation of coagulasepositive staphylococci on V.J. Agar is based on mannitol fermentation and tellurite reduction (5). V.J. Agar is specified in the standard methods for examination of cosmetics (4, 6), pharmaceutical articles and nutritional supplements (2). In addition, the formulation complies with recommendations by the USP for microbial limit testing (2).
Casein enzymic hydrolysate and yeast extract provide nitrogenous compounds, vitamin B complex and other growth nutrients. Dipotassium phosphate provides buffering to the medium. During the first 24 hours of incubation, contaminating organisms are almost inhibited by tellurite, lithium chloride and high glycine content. The effect of inhibitors on S. aureus is reduced because of the presence of mannitol and glycine. Coagulase-positive staphylococci reduce potassium tellurite to metallic free tellurium and thus produce black colonies surrounded by yellow zones. This yellow colour is due to phenol red indicator that turns yellow in acidic condition due to the fermentation of mannitol. If mannitol is not fermented, yellow zones are not formed. Also the color of the medium around the colonies may even be a deeper red than normal due to utilization of the peptones in the medium. Prolonged incubation may result in the growth of black coagulase-negative colonies.
Storage and Shelf-life:
Store below 30°C in a tightly closed container and the prepared medium at 2 - 8°C. Use before expiry date on the label.
References:
1. Vogel R. A. and Johnson M. J., 1960, Public Health Lab. 18:131.
2. United States Pharmacopeia, 2008. United States Pharmacopeial Convention, Inc., Rockville, Md.
3. Zebovitz E., Evans J. B. and Niven C. F., 1955, J. Bacteriol., 70:686. 4. FDA Bacteriological Analytical Manual, 2005, 18th Ed., AOAC, Washington, D.C.
5. MacFaddin J. F., 1985, Media for Isolation-Cultivation-Identification-Maintenance of Medical Bacteria, Vol. 1, Williams & Wilkins, Baltimore, Md.
6. Curry A. S., Graf J. G. and McEwen G. M., (Eds.), 1993, CTFA Microbiology Guidelines, The Cosmetic, Toiletry and Fragrance Association, Washington, D.C.